Phase control in electrical transmission



E. l. GREEN ETAL 2,284,612

Filed Jan. 6, 1940 10 Sheets-Sheet 1 N MN( ATTORNEY QQ m. ...El

/NVENTQRSLE GREEN BV y' am E NQ @Q May 26, 1942.

PHASE CONTROL IN ELECTRICAL 'TRANsMrssIoN l0 Sheets-Sheet 2 May 26, 1942. E. l. GREEN rs1-*AL l PHASE CONTROLv IN ELECTRICAL TRANSMISSION Filed Jan. 6, 1940 May 26, 1942. E. l. GREEN Erm.

PHASE CONTROL IN ELECTRICAL TRANSMISSION Filed Jan. 6, 1940 10 Sheets-Sheet 3 .EL GREEN NVENTORS. MMO/VK 1 1 l,

AToR/VEV May 26, 1942. E. l. GREEN ET'AL v PHASE CoNTRoL IN ELECTRICAL TRANSMISSION Filed Jan. 6, 1940 10 Sheets-Sheet 4 E. GREEN /NVE/VTORS- AT TORNEV May 26, 1942. E. l. GREEN ETAL PHASE CONTROL IN ELECTRICAL TRANSMISSION Filed Jan. 6, 1940 10 Sheets-Sheet 5 E. l. GREEN /NVEA/roRs. ,v MOM( A T TORNE V E. Yl. GREEN E-rAL 2,284,612

Filed Jan. e. 1940 1o sheets-sheet s May 26, 1942.

PHASE GONTROLl IN ELECTRICAL TRANSMISSION May 26, 1942. E. l. GREEN ET AL PHASE CONTROL IN ELECTRICAL TRANSMISSION Filed Jan. 6, 1940 lO Sheets-Sheet 7 Ilm El. GREEN `/A/L/EA/Tomi N MOM( May 26, 1942. E. l, GREEN ET AL PHASE CONTROL IN ELECTRICAL TRANSMISSION Filed Jan. 6, 1940 lO Sheets-Sheet 8 Trb fl E./. GREEN A QW um u W /Nl/E/VTORS.' MMO/VK ATTORNEY BVEI,

10 Sheets-Sheet4 9 E. l. GREEN ETAL PHASE CONTROL IN ELECTR-ICAL TRANSMISSION Filed Jan. 6, 1940 May 26, 1942.

Filed Jan. s, 41940 Al0 Sheets-Shed'.l 10

.www mow men E. GREEN WEA/mns.- -IB N. MON/f A TTORNE V Patented May 276, 1942 PHASE CNTBOL IN ELECTRICAL TRANSMISSION Esau Ibbomn Green, snm rims, N. J., and Newton Monk, New York, N. Y., assignors to American Telephone and Telegraph Company, a corporation of New York Appacason January s, 1940, serial No. 312,792

14 Claims.

' This invention relates to methods and means for compensating for phase changes of a transmission line and to their application for any desired use, examples being for automatically controlling voltage regulation and load distribuat two points separated by an appreciable length of line.

Another object is to maintain identical phase of the carricr frequency of a plurality of common frequency broadcast stations or identical phase relationship of the currents or voltages in diil'erent systems.

Another object is to provide a simple and continuously operating system for controlling in acchang'es which affect principally the resistance and to a lesser extent other circuit constants.

cordance with phase relationship the voltage reg` A further object is to provide means for automatically carrying out the above-named objects.

Frequency synchronization in electrical sys-Y tems is a usual requirement, while phase synchronization is not' always necessary. This invention embraces the general problem of maintaining constant phase shift for a substantial length of line and is directed to methods and means for automatic compensation of variations in phase change of on lines, thereby securing constant phase shift.

In applying this invention to the control 'of other systems a phase compensated pilot line or the equivalent is generally .employed in determining the difference in phase of a voltage delivered to the phase compensated circuit at one point with that of a local source at another point by connecting a phaseangle meter or other suitable device between the local sourcel and the pilotY line reaching this point. V-

Thephaseshiftcharactesticofalineofappreciable length, for instance,such asmight' be used as a pilot line, does not remain constant with time but varies with temperature and vweather conditions. For cable circuits the `variationinphaseshiftisduetotemperature For open-wire lines the changes in phase result largely from changes in leakage conductance of the insulators, although temperature eiects are also important.

Compensation for variations in phase change of a line as disclosed herein vmay be brought about by three diierent methods: (l) Direct current method;V (2) Alternating current method; and (3) Carrier current method. f I

The direct current method is primarily applicable tocable circuits. Further, the arrangement is best adapted to cable circuits operating at relatively low frequencies. Thismethod comprises maintaining constant phase shift for subj stantial length of line circuits by taking advantage of the relation of the phase shift to resistance. This relationship may be mathematically expressed. For cable circuits at low frequencies, the propagationconstant is given by theY following approximate formula:

Since the capacitance C changes very .little with temperature, it will be seen that the change in the phase shift results largely from the change in resistance.

In the first arrangement here shown, the varlae' the same conditions, such as several circuits in*L I the same cable, the arrangement applicable to one circuit may be slightly modified and one of the circuits which may be termed a pilot cir"-v cuitmaybeusedtocorrectthephwevarlations of allot the circuits. This avoids duplication of the compensating apparatus for each circuit.

The alternating current method is not limited to making approximate phase change correction onthebasis ofvarlation only. Item-`v ploys analternating currentinmaintainlng constant phase shift for substantial lengths ofline jcircuits byanentwhichobtaimsubstannally perfect correction of phase change and is applicable alike to cable or to open-wire lines.

The carrier current method comprises maintaining constant phase shift for substantial lengths of line circuits by an arrangement which also obtains substantially perfect correction of phase change for whatever cause and is applicable alike to cable and to open-wire lines and particularly to high frequency operation. This arrangement employs a carrier current which is modulated by a W frequency current at the ltransmitting station for use/in effecting the phase shift compensation of a line.

With any of the three `methds the line can, with suitable arrangements, be used simultaneously for other purposes.

A description of a number of embodiments chosen for illustrating this invention in which reference is made in each case to the accompanying drawings follows:

Figure 1 shows an arrangement employing drect current for the automatic compensation of phase change variations of a line in which advantage is taken of the relation of the change in phase to the change in resistance.

Fig.- 2 shows more in detail the automatically operated adjusting mechanism of Fig. 1 and of other figures Fig. 3 shows a possible type of circuit arrangement for the phase adjuster of Fig, 1 and of other figures.

Fig. 4 shows a typical circuit arrangement of the phase angle meter of Fig. 1 and of other fig- ,nism is used to connect one phasel after another.

Fig. 12 shows'an arrangement employing al-` ternating current for automatically compensating a three-wire pilot line for phase change variations between two points arranged for simultaneous phase comparisons of the voltages of all three phases of a power system or syste'rnsy at the two points between which the three-wire compensated pilot line extends. 'Suitable coupling means are employed to connect the pilot line with the power lines to simultaneously comparev all three phases of the power system or systems.

Fig. 13 shows an arrangement employing carrier current for automatic compensation of phase change variations of a line in which a pilot current consisting of a carrier current modulated by a low frequency current is transmitted between,`

two points in opposite directions over the line and illustrates how the arrangement can be emhybrid coil type of line retomatically controlling the voltage or load at a powerstationin accordance with the phase indications transmitted over a compensated line.

Fig. 8 shows an arrangement employing a1- ternating current for the automatic compensation of phase change variations of a line in which a single pilot current ofthe same frequency is transmitted between two points in opposite directions over two different lines. 'Ihis ligure also shows how the arrangement may be employed to obtain, comparisons of the phase relation'of voltages at the two different points.

Fig. 9 shows an arrangement employing alternating current for'the automatic compensation of phase change variations of a line-using filtersl in whichpilotcurrents of two different frequencies are transmitted between two points in opposite drectionsjover the same line. This figure also shows' h'ow'the arrangement may be employed to obtain :comparison of the phase relationship'lof` voltages vat the two different points.

Figi-10- sh'ows.-'a filter type line repeater. l,

' 1" shows Aan arrangement employing alg'current' for automatically compensatgna` two-wireline lfor phase change variations between twozpoints arranged for making successive iphase comparisons of the voltages of` all three"=.phas`estoi?I a power system or systems at the ployed for making comparisons of the phase relationship of independent voltages at two different points.

Fig. 14 shows an arrangement employing carrier current for automatic compensation of phase change variations of a line in which the pilot current in one direction consists of the carrier current modulated by a low frequency current and in the opposite direction of .a harmonic frequency of the carrier current, thus using different pilot frequencies for transmission in opposite directions over the line and also illustrates how thisarrangement'can be employed for making comparison of the phase relationship of independent lvoltages at two different points.

Fig. 15 shows an arrangement employing carrier current for automatically compensating ay pilot line for phase change variations between two points of a power system in which the carrier current modulated by the power current is transmitted over the automatically compensated pilot line and in which synchronous switching mechanism is used to connect in one phase after another of the power current formaking phase comparisonof the power current or voltages at two different points.

Fig. 16 shows an arrangement lemploying carrier current for automatically compensating a pilot line for phase change variations between two points in which three different frequency carrier currents modulated by the power current of the different respective phases of the power;

system are transmitted over the automatically compensated pilot line for simultaneously making rov twoppointsffbetweeri' which. the compensated pilot "extends Synchronous switching mechaphase comparisons of the power currents or voltages. s.

Fig. 17 shows an arrangement employing carrier current for automatically compensating a three-phase pilot line for phase changey variations between two points arranged for making phase comparison of the currents or voltages of all three phases of a power system at two different points in which three) different carrier'curl rents are modulated -by the power current of the different respective phases of the power system and are transmitted over the respective phases of the automatically compensated three-phase pilot line for simultaneously making phase comparison' of the power'currents or voltages.

Fig. 18 shows an arrangement employing carrier current for automatically compensating ,one phase of a three-phase power line for phase change variations `between points in which the compensation is achieved by transmitting a modulated carrier current in opposite directions over the compensated one phase of the power line wire and arrangements at one DOint for making comparison of phase relationship'of the currents or voltages at the two different points.

Fig. 19 shows an arrangement employing carrier current for automatically compensating one phase of a three-phase power line for phase change variations in which the compensation is achievedy by transmitting a modulated carrierv current in one direction and in returning a harmonic frequency thereof over the compensated one phase of the power iinewire and arrangebridge |30, the resistance |3| which is madeY ments at one point for making comparison of Y wires between the two points and in which syn# chronous switching mechanism is used to connect in one phase after another of the power circuit for making phase comparison of the power current or voltages of the three-phase system at the two diierent points.

Fig. 21 shows an arrangement employing carrier current for automatically compensating all three phases of athree-phase power line for phase change variations between two points in which Vcompensation is achieved by transmitting three diiferent suitably modulated carrier currents in opposite directions over the compensated power line wires of each phase between the two points and arrangements at one point for making comparison of phase relationship'of the current or voltages in the diierent phases of the power circuit at the two different points.

Fig. 22 shows an arrangement employing carrier current for automatically compensating one phase of a three-phase power line for phase change variations between two points in which comparison is achieved by transmitting threeVdifferent modulated carrier current frequencies in opposite directions over the compensated one phase ofapower line between the two points and arrangements at one point for simultaneously making comparison of the phase relationship of the currents or voltages of the diiferent phases of the three phase power circuit at the two different points.

Fig. 23 shows an arrangement employing carrier current for automatically compensating for nesting emplyng'commnn frequency carriers.

Similar reference characters refer to correj spending parts in the different figures ofthe noting current for transmission over pilot line |00. Inh'ansmisionover theline the current or voltage uxxdergoesartainphase shiftand by mieansofaphaseiuijuster lilthephaseshift -maybeneutraliaed completelyorpartially. 'I'he change variations of pilot linesinterconf dio broadcast stations for .achieving phase mehronmtion of broadcast transmitters pilot line |00 itself lcom-prises one arm of the equal to the average resistance of the line forms a second arm, while resistances |32 and |33 are the other arms. Between the line and the resistance |3| is connected a variable resistance |35 for balancing the bridge. Across the opposite points of the bridge is connected a polar relay |35.

When the resistance of the line |00 changes, this relay closes either its upper contact |31 orits lower contact |38, depending upon the direction of change. rAutomatic adjusting mechanism Ill then adjusts the resistance |35 until the bridge is once more balanced. e

The automatic adjusting mechanism lll, shown more in detail in Fig. 2, is such Athat its motor |4| rotates in one direction when the upper Contact |31 of the polar relay is closed and inthe other direction-when the lower contact |38 commencing on'page 522.'

is closed. The motor of the adjusting mechanism is mechanically geared to the variable resistance and also to the phase adjuster |50 so that in addition to rebalancing the bridge, the

adjusting mechanism serves also to actuate thephase adjuster to compensate for the change in Y phase which accompanies the change in line resistance. SinceY only a single frequency is involved, the phase adjuster may be quite simple.

The phase adjuster. for example, may be simply a variable condenser or inductance or it may be of a somewhat more complicated type. .A possible type of phase adjuster is shown in Fig. 3. The total range of the phase adjuster |50 must be as great as the maximum variation in the Two or more sec phase shift of the line |00. tions like that shown in Fig. 3 may be used in series in case suiiicient phase shift is not ob-f tained in a single section.

In the, circuit connecting the line with the bridge, a nfilter |20 is provided to keep the alter'- nating current from the source l0 and other superfluous frequencies out of the bridge circuit. CondenserslZl and |22 are provided to isolate the receiving apparatus at station B from the bridge. of the source |0.the same at the receiving-point as that at the point of origin a phase compensator |10 may be inserted in the line to provide a constant phase shift opposite to the net phase shift introduced by the line and the phase adjuster |50. It is possible in most cases, however, to omit -the phase compensator |10 and if necessary to calibrate the receiving apparatus accordingly. In order to determine the amount of phase shift to be provided by the phase compensator |10, it is necessary to know the phase shift of the line. This may be obtained from calibrations based on the fundamental constants of the circuit or measurements of the phase change may be made by -employing one of the various methods discussed by Messrs. Nyquist and Brand in an article entitled The measurement of phasedistortion, published in the Bell System Technical Journal of July 1930, volume IX, With the exception of the open and Yshort-circuit impedance method, which is applicable only to lines which are electrically short, all of the methods discussed by 'Messrs Nyquist and Brand require that both ends of the circuit be Yavailable at the same location. This is .usually achieved by employing two similar circuits, looping them together at the distant end and dividing the measured phase change by two.

Where two similar circuits are not available, theV Where it is desired to have the phase method described in the copending application of Messrs. E. I. Green and N. Monk, Serial No. 259,494, filed March 3, 1939, now U. S. Patent No. 2,214,130 of September 10, 1940, may be employed.

The compensated pilot line provided by the arrangement shown in Fig. 1 may be used for indicating the diiference in' phase between the currents or voltages of two sources at stations A and B, respectively. At station A a source and at station B a source 2|0 separately provide alternating current power of the same frequency. The separate sources may be supplying power to one transmission system or to two diiferent transmission systems. In supplying power to the same or different systems the load distribution or its Adivision as between the sources may be controlled in accordance with the phase angle displacement between the respective power currents or voltages. By the arrangement here shown, the phase angle between the two respective currents or voltages is indicated and variations in quently described herein. One line circuit which might be termed a pilot circuit would be used in connection with a single bridge and rebalancing mechanism and this mechanism would serve v to correct the phase on'a number of other circuits. 4

Fig, 5 shows an arrangement for obtaining substantally perfect correction of variations of phase change introduced in a transmission line.

This arrangement is applicable alike to /both' cable and ,open-wire lines and to low or Vhigh frequencies. Fig. 5 shows an` arrangement employing alternating current for the automatic compensation of` phase change variations of a. line in which the alternating current method above mentioned is employed. A pilot currentv of the same frequency is transmittedbetween two points in opposite directions over the same line, using hybrid coils, for making comparisons of the phase relationship of currents or voltages existing at the two different points. Stations A and B at distant points include the sources |0 and 2|0, respectively, for generating alternating phase difference between voltage V1 and V2 genj erated by the sources |0 and 2|0 at stations A and B, respectively. The means for utilizing the received frequency after phase correction is shown in Fig. 1 asa phase angle meter 200.

'I'his meter may be used to indicatethe difference in phase between ythe voltage V"i delivered by the phase correcting mechanism `and the local voltage V2.

The indications of the phase angle meter or of a recorder may be used as a guide for the control of load distribution between two generating stations whose voltages are being compared. Such control may be accomplished through manual adjustment of tap changers or other devices well currentof low frequency and by the arrangement shown the phase angle between the two currents or voltages is indicated and can be employed for purposes of load control. In this arrangement variations in the phase shift of the line |00 are automatically compensated through the lagency of a balanced rectier and relay 30 which causes the operation of adjusting mechanism which in turn controlsphase angle adjusters 50 and 60 in accordance with the action of the balanced rectier 30. Alternating current from source |0 at station A after passing through current limiting resistances the phase adknown in the power industry. It is also possible,

as discussed later, to utilize the phase relationship between the voltages of the two stations to obtain automatic voltage control.

The phase angle meter 200 may be of any well-known type such as, for example, the ',Iuma phase meter diagrammatically shown in Fig. 4.- This arrangement consists of a fixed coil 20| within which two similar coils 202 and 203 rigidly set at right angles to each other are allowed to rotate freely. The voltage V1 is passed through the fixed coil 20| While the voltage V2 is passed through both of the rotatable coils 202 and 203 in quadrature,l this being accomplished by inserting aresistance 204 in series with coil 202 and an inductance 205 in series with coil 203. If desiredrthe phase angle meter 200 may be a phase angle recorder such as,- for example, one of the type shown in Fig. 4 of the article by Forbes and `Searing entitled Voltage and load control pub lished in Electrical Engineering, June 1934.

When it is desired to obtain constant phase shift on several line circuits which are, exposed to the same condition, for example, several circuits of the same cable, it is possible to apply the arrangement of Fig. 1 in a slightly different way.

` This is also applicable to arrangements subsejuster 50 and hybrid coil |20 having a balancing network |2| is transmitted over the line |00 to station-B where a part of the incoming energy is taken off throlugh hybrid coil 220 having a balancing network 22| and after being amplified in amplifier 290 which should be designed for constant phase'shift is returned to the line through aL conjugate pair of terminals of hybrid coil 220.7 At station A a part of the returnedv energy is withdrawn from the line |00 through hybrid coll 20 and after passing through phase adjuster E0 is applied to the common grid connectionof the balanced rectifier 30. Also; alternating current from thelsource I0 after passing through current limiting resistances I2 and phase compensator 10 is applied to the balanced rectier 30. 'I'his rectifier arrangement has the wellknown property that when the two inputs differ in phase by 90 degrees, or more generally by 1hr/2 where n is any integer; the outputs of the two rectifier tubes will be equal. This is true regardless of the relative amplitudes of the two inputs. If, however, the 90 degree Aphase relationship between the inputs is altered, the polar relay 30 will operate in one direction or the other dependingon thedirection of the phase differ-` ence. The operation of this polar relay controls the operation of adjusting mechanism 40 which may be similar to that shown in Fig. 2. 'I'he operation of this adjusting mechanism in turnsimultaneously adjusts the phase adjusters 50 and 60 to restore the condition of balance. Assuming that the changes inphase shift are the same for either direction of transmission over theline, the phase shift introduced by the phase adjusters 50 and,60 should be made equal. It

will then be evident that as long as a condition fof balance holds at station A, the phase of the received currents or voltages at station B will always have the same relationship to that of station A. The total range of adjustment of each of the variable phase adjusters 50 and 60 must be as great as the maximum variation of the phase shift of the line. The phase compensator 'I0 may be employed to obtain the desired 90 degree phase relationship between the two inputs to the balanced rectifier 30. The phase angle difference between the' alternating currents or voltages generated by sources and 2|0 at the two stations A and B, respectively, is l e shown by the phase angle meter '200. Alternating current from the source I0 after being transmitted over the compensated line |00 is impressed on one side of the phase angle meter 200 and alternating current from the source 2|0 is impressed upon the other side of the phase angle meter. High resistances 2|| may be inserted in the leads from the compensated line at station B to provide the necessary impedance to prevent losses to the line by the connected terminal apparatus. A phase compensator 210 may be inserted to provide a phase shift opposite to the phase shift introduced by the line so that the total phase difference between the currents or voltages of the sources |0 and 2|0 may be read directly. However, this phase compensator 210 may be omitted and if desired the receiving apparatus calibrated accordingly. In order to determine the amount of phase shift to be provided by the phase compensator 210, it is necessary to know the phase shift of the line. may be obtained from computations based on the fundamental constants of the circuit or measurements of the phase change may be made by employing one of the various methods referred to in describing Fig. 1.

Fig. 6 diagrammatically shows the well-known hybrid coil type line repeaterwhich may be used in the line circuit |00. One or more of these or other suitable types of repeaters may be used in various arrangements here described.

If it 'is desired to employ the phase'compari-v sons obtained with the arrangement of Fig.'1 or `Fig. 5, as a basis for automatically controlling the load distribution on a system, this can be done by means of the arrangement shown in Fig. 'I which may be substituted for the phase angle meter shown in Figs. 1 and 5, the circuit connections being made at the point indicated which are well-known. Such' an automatic arrangement may be used in conjunction with an arrangement for phase angle control between two intermediate power systems.'

Fig. 8 shows a compensation arrangement em-4 ployed to obtain comparisons of the phase relationship of currents or voltages at tw'o different points. Stations A and B at distant points are provided with sources I0 and 2|0, respectively, for generating alternating current of the same frequency.V In this arrangement the variations in the phase shift of the outgoing line circuit |60 and the returning line circuit |0| are automatically compensated and held substantially constant through the agency of a balanced rectifier and relay which causes the operation of an adjusting mechanism 40 which in turn controls the phase angle adjusters 50 and 60 in ac- This' Y upon the phase angle meter.

2|| in the leads vfrom the compensated line to ploying two pilot lines, an outgoing and a returning pilot line. This shows an.arrangement also employing alternating current forgthe automatic compensation of phase changefvariations of a line in which a single pilot current of the same frequency is transmitted between two points in .Opposite directions over two` different lines and fit also shows how the arrangement may be emcordance with the action of the balanced rectifier 30. Alternating current from the source I0 at station A passes through current limiting resistances and phase adjuster 50 for transmission over the line |00 to station B and back over the return line |0| to the phase adiuster B0 and thence to the balanced rectifier 30 at station A. The balanced rectifier 30 and the adjusting mechanism l0 are similar to those shown in detail in Figs. 5 and 2, respectively. Current vfrom the source I0. at station VA is also fed through current hunting rsistances I2 and phase compensator 10 tothe balanced rectifier 30. The operation of the balanced rectifier causes the adjusting mechanism 40 to simultaneously operate the phase adjusters 50 and 60 so as to restore the condition of balance and thus automatically compensate the lines |00 and |0| interconnecting stations A and B. The phase compensator 10 may be employed to obtain initially the desired degree phase relationship between `the two inputs to the balanced rectifier 30. The phase angle adjusters 50 and 00 must be of ample range to compensate for both line's. The action of the phase adjusters and the balanced rectifier Vis similar in this arrangement to that described more in detail for Fig. 5. The phase angle difference between the alternating currents generated at the two stations is shown by phase angle meter 200. Current from source |0 after beingv transmitted over compensated line |00 is impressed upon phase angle meter 200 at the same 4 time that current from source 2|0 (is impressed High resistances the terminal apparatus prevent this apparatus from affecting transmission over the pilot lines.

A phase compensator 210 may be inserted to provide a phase shift opposite to the phase shift introduced bythe pilot line |00.

In the arrangements so far described, the pilot currents employed are the same frequency for opposite directions of transmission or in other words, for both directions of transmission. However, in some instances it may be advantageous v to employ different frequencies for opposite directions of transmission. Fig. 9 shows an arrangement in which this is possible. This figure shows an arrangement employing alternating current for the automatic compensation of yphase change variations of ra line using filters, in lwhich a pilot current of different frequencies is transmitted between two points in opposite directions over the same line and this figure also shows how the arrangement may be employed to obvtain comparison of phase relationship of currents locally from the original source. In this case the phase adjusters 50 and 60 are not alike and each must correct for phase change variations at the particular frequency transmitted through it. While it would be possible with this arrangement to use hybrid coils to separate the oppositely directed currents, the use of different frequencies in opposite directions makes it possible to employ filters instead of hybrid coils at the p terminals and repeaters. Stations A and B at distant points are" provided with sources I and 2|0, respectively, 4for generating alternating currents of frequency f1 whose phase relationship is to be compared and by mea s of the arrangement shown the phase angle between the two currents or voltages is indicated. In this arrangement variation of the phase shift of line |00 is automatically compensated` through the agency of the balanced rectifier and relay 30 which cause operation of adjusting mechanism 40 which in turn controls phase angle adjusters 50 and 60 in accordance with the action of balanced rectiiier 30. Alternating current from source |0 at station A after passing through current limiting resistances Il., phase adjuster 50 and filter 5| transmittingfrequency f1; the same as that from source I0, is transmitted over'7the line |00 to station B where a part of the incoming energy is taken oil through filter 25| transmitting frequency f1 and after being amplified in amplifier 290 is transmitted to harmonic generator 260 from which a harmonic of this frequency is derived, for example 2f1, whichis transmitted through filter 26| back to the line |00 and .to station A. At station A the returned energy is withdrawn from the line |00 through filter 6|,

l transmitting frequency l2f1 andgafter passing through phase adjuster 60 is applied to the common grid connection of balanced rectifier' 30. Also current, from the source I0 is transmitted throughcurreni; limiting resistances I2 to harmonic generator-20 where a harmonic frequency '2h is derived which is transmitted by filter 2| passing frequency 2f; through phase compensator to the balanced rectifier 30. The operation of balanced rectifier 30 controls the operation of adjusting mechanism 40 which causes phase adjusters 50 and 60 to operate to restore the-conknown in the communications and signalling art.

Fig. 1l shows an arrangement employing a1- ternating current for automatically compensating a Vtwo-wire line for phase change variations between two points arranged for making successive phase comparisons of the voltages of al1 three phases of a power system or systems. At the two points between which the compensated power line extends, synchronous switching' `protective condensers and resistances -to keep phase angle comparisons.

dition of balance and thus automatically compensate the line |00 for changes in phase shift.'

In this arrangement a harmonic oi' the-pilot current frequency is impressed upon both sides of the balanced rectifier instead of `a 4current of the original pilot frequency. Current from source I0 after being transmitted over compensated line |00 is also impressed upon one side I-oi phase angle meter 200 and current from source 2|0 is impressed upon the other sideof the phase angle meter 200, andthe phase angle difference between the current or voltages generated at the two stations A and B is shown by this phase angle meter.-r High resistances 2|| are in the leads connecting the compensated line and the terminal apparatus to limit transmission losses due to the bridged apparatus. A phase compensator 210 may be inserted to provide a.V phase -shift opposite to they phase vshift introduced'by the line.

Fig. 10 shows the well-known illterY type line repeater, particularly applicable for use with the arrangement shown in Fig. 9. One side of the repeater transmits frequencies of f1 and the other side frequencies of 2f; or any other harmonic nfi of the fundamental frequency f1 which m y' be selected or used. Such repeaters are ll th power currents oi the pilot line and its terminal apparatus. At station A the three-phase power circuit 5|0 is tapped by three leads through protective condensers 52|, 522 and 523 and respectiveseries resistances 525, 526 and 521 in each lead to connect theV different phases A,

=B and C'with the synchronous switch 3|0 which 30 sating equipment 99 with the compensated pilot connects through the automatic phase compenline |00 leading to station B. At station B the three-phase power circuit 6|0 is connected by three leads through protective condensers 62|, 622, 623 and respective series resistances 625, 626

and 621 in each lead to connect the different phases A, B and C of the power circuits through respective phase angle meters 200A, 200B and 200C. 'I'hese are in turn connected with the compensated line 00 through the automatic phase compensating equipment 299 and the synchro-pv nous switch'4l0. The synchronous switches at the .two stations A and B may be of any wellknownitype and operated in synchronism by any suitable means, not shown in the drawing,to con.- nect the different corresponding phases of the power systems at the two stations for making Current from each phase of the power system vat station A, after being transmitted over the compensated line |00, is successively impressed upon the phase angle meters 200A, 200B and 200C'of the respective phases A, B, and C for making the phase comparisons with the currents or voltagesof the power system at station B which is also successivelyimpressed upon the phase angle meters, In this arrangement the automatically compensated pilot line |00 and theterminal apparatus at each station makes it possible to compare the cession as shown in Fig. 11.

phase angle difference of the currents or voltages in the respective phases of the two power systems vat any time by voperating the synchronous switches at the two stations manually or 'they may be continuously operated automatically at such a speed as to/permit continuous readings-on the phase angle meters;

In the arrangements so far discussed, two-wire' pilot lines have been employed without recourse to multi-channel carrier current operation and phase comparisons were thus limited to Vcomparing single phase circuits or sources at a vtime so that when polyphase systems were involved the comparisons were. necessarily made in suc- .lig. 12 shows anar-rangement for This figure shows .an arrangement employing alternating current for automatically compensating a three-wire pilot line for phase change variations between two points arranged for simultaneous phase comparisons of the currents or voltages of all three phases of a power system or systems at the two points between which the three-wire compensated pilot line extends. Suitable coupling means are employed at the points to connect the three-wire pilot line with the power lines for simultaneously making phase relationship comparisons between all three phases of the power system or systems. One phase of the three-wirepilot line 'automatically controls the compensation of all Aof the three phases of the pilot line as, where the circuits of the three-wire pilot line are close together, one circuit may adequately serve to govern the compensation of all pilot line |05, the compensating apparatus is controlled by` one phase of the pilot line-as stated above. Alternating current from one phase of the power-line, after transmission over one phase` of the pilot line and through three-phase lter 630, transmitting frequency f1, is taken o at station B through modulator 264 and by means of oscillator 265 is stepped jup to a different frequency Vf2 and returned through filter 263 to the one phase of the pilot line and then over it to station A. The return frequency f2 is passed by filter 63 to demodulator 64 from which current of frequency f1 is transmitted to the balanced rectifier 30, while current of frequency f1 from the power line at station A is passed through filter 22 which transmitsv frequency f1 to phase cornother suitable frequency. Subsequently an arrangement will be described herein in which the power lines are used for transmitting the compensating currents and for operating the compensating apparatus as well as transmitting the power current. o

It may happen in some instances' that'the line is not adaptable to transmitting low frequencies or it may be desirable for some other`reason to employ high frequency transmission over the pilot line.

Fig. 13 shows an arrangement employing carrier current for automatic compensation of phase change variations of a line in which a pilot current consisting of a carrier current modulated by from source I0 modulates in modulator 9| a carrier current generated by oscillator 90 and the upper (or lower) side-band frequency is transmitted through filter 92, phase adjuster 50, hy-

line |00. As the pilot line |00 here shown is a vsection of a telephone circuit, the above-mentioned currents in reaching the pilot line pass through high-pass filter ||8 which prevents Athe telephone signals on the pilot line reaching the terminal apparatus. -The telephone circuit is continued through low-pass filter H9. At stai tion B the pilot vline connects with the terminal apparatus through high-pass filter 2|6'and with the telephone circuits through low-pass filter 2|9. At station B a part of the high frequency current is ampliiiedand returned by means of hybrid lcoil 220 over the pilot line |00 to station A where it passesthrough phase adjuster 60, demodulatorv 00 and filter 65, vwhich passes only currents of the same frequency f1 as that of source I0 to the balanced rectifier 30. The low frequency source of current I0 transmits frequency f1 through the phase compensator 10 to the other side of the pensator 'I0 and to the balanced rectifier 30.

The balanced rectifier controls the operation of the adiusting mechanism 40 which in turn drives the phase adjuster 80 which-operates on all three phases of the pilot line |05, thus automatically4 The pilot line and phase adjuster should have a phase compensating each circuit of the pilot line.

to the automatic compensating equipment. At

station B phase angle Vmeters 200A, 200 B and 200C are connected through phase compensator 610 with the different phases of the compensated three-phase pilot lines |05 vand through con- -densers 62|, 622 and 623 to the corresponding phases of the power system 6|0. The phase Aangle meters, therefore, show at all times the phase relationship of the currents or .voltages in the different phases of the circuits of the power systems 5|0 and 6|0. The return frequency fz over one of the phases of the pilot lines |05 may have a harmonic relationship to the frequency of the power line current or it may be of anyl balanced rectifier Il. The balanced rectifier controls the adjusting mechanism 4l which in turn operates thev phase adjusters 50 and 60 to automatically compensate the pilot lines |00 in a'manner similar to that explained more in detail in the vdescription of Fig. 5. At station B connection is made with the compensated line through high resistance 2| phase, compensator 210, demodulator 26|, filter 265 passing a fre- .quency f1 to the phase angle meter 200. The

phase angle meter 20| is' also connected to a source of current 2|0 having a frequency fr whose phase relationship is to be compared with that of the current from source '|l. Thus the phase angle meter A200 indicates the phase relationship between these two'sources. This figure andvsubsequent figures show various arrangements employing carrier current for compensating a pilot line and various applications of such compensated lines to-indlcate phase relationship and to compare the phase relationship of the currents or voltages of power systems.

While the .carrier current arrangement shown in Fig. 13 as in subsequent figures shows two phase adjusters at station A, it is possible to use only o ne adjuster in certain of the carrier current brid coil. |20 and high-pass filter IIB to the pilot i may be 2(f1+f2).

arrangements. Fig. 12shows a single phase adjuster and the arrangement inthat figure may be considered a partial carrier current system in that the returned current modulatesafcarrier change variations o f a line in which the pilot current in one direction consists of the carrier current modulated by a low frequency current and in the opposite direction of the harmonic frequency of the carrier current thus using carrier currents of different frequencies for the transmission in opposite directions overthe line and also Iillustrates how this arrangement can be employed foremaking comparisons of phase relationships of independent currents or voltages at two different points. Fig. 14is similar to Fig. 13 except'that the returned current is a harmonic of the transmitted current and filters instead of hybrid coils are employed for separating the two` currents. In .this arrangement a frequency f1 supplied by source I modulates in modulator 9| a carrier frequency f2 supplied'by oscillator I90 and the upper (or lower) side-band is transmitted by lters 92, 93 and H8 to the pilot line |00. upon reaching station B istransmitted by lters 2|8 and 25| and amplifier 290 to harmonic generator 260. The returned harmonic frequency It is transmitted back to station A through lters 26|, 2|8, ||8 and 6| to phaseadjuster 60 and to one side of the balanced rectifier 30.. At station A part of the upper sideband current from modulator 9| andvfilter 92 is impressed upon harmonic generator 20 and the harmonic component having a frequency 2(11-l-j2) the same as that returned over the pilot line |00, yis passed through'fllter 2| and phase v compensator to the other side ofthe balanced rectifier 30. The balanced rectifier 30 in turn controls the operation of the adjusting mechanism 40 which operates phase-adjusters 50 and 6|!v `to automatically maintain compensation of the pilot line |00. At station B a part of the transmitted current is also passed through high re'- angle meter at station B.

Fig. 15 shows an arrangement employing carrier current for automatically compensating a pilot line for phase 'change variations between two vpoints of a power system in which the carrier current modulated by the power current is transmitted over the automatically compensated pilot line and in which synchronous switching A source yof current 2 I0 having the fre-v The current ment 99 and 299 at stations A and B, respectively, may be the same as shown in other draw'- ing gures such as Fig. 13 or Fig. 14, the compensating apparatus being shown within the rectangulardot-dash line in those figures. A carrier frequency f: is supplied yby oscillator 90 at station A. The synchronous switching mechanisms IIB and 4|0 at stations A and `1? respectivelyl which connect with the poly-phase power systems 510 and 6|0, respectively, each transmit the same frequency f1, at the two stations-and with the compensated pilot line may besimilar to the arrangement shown in Fig. 11. At station B phase angle meters are shown for each phase of the power circuit 200A, 200B and 200C for phases A, B and C, respectively of the power systems. By means ofthe synchronous switching arrangement the relative phase difference between the corresponding phases of the two systems may be successively measuredr on the respective phase angle meters. One phase angle meter might be used by positioning it in the common circuit between the automatic phase compensating apparatus 299`and the synchronous switch 4|0. v

Fig. 16 showsan arrangement employing carrier current for automatically compensating a pilot line for phase change variations between two points in which three different frequency carrier currents modulated bythe power currents of the different respective phases of the power system are transmitted over the automatically compensated pilot line for simultaneously making phase comparisons of the power currents or voltages. Each phase of the power circuit 5|0 employs individual automatic phase compensating equipment 99Al 99B and 99C, respectively. This compensating equipment may be similar to that shown within the dash-dot line at the station A of Fig. 13 or 14. Carrier vcurrents of different frequencies fz, f3, and f4 are supplied by sources 90A, 90B and 90C, respectively, for each of the dilerent phasesof the power system having a frequency-l1. Connection is made with the pilot line |00 through suitable lters 93A, 93B and 93C foreach of the three phases, respectively. 'If the automatic compensating arrangement is similar to that shown in is similar to thatshown in Fig. 14,4 these filters transmit frequencies (f1 +f2) and 2( f1+f2),(f1+fs) and 20H-Hs), and (f1-H4) and 2dr-H4), respectively. At station B the automatic phase compensatingequipment 299A, 299 and 299Cv for the respective phases of the power system may be simi- .lar to that shown in Fig. 13 or 14. Respective illters' 293A, 293B and 293C connect the automatic mechansim is used to connect in o'ne phase after another of the power circuits for making Aphase comparisons of the power currents or voltages at two different points. This ,gure illustrates the yphase compensating equipment of the different phases with the compensated pilot. line |00.

Phase angle meters 200A, 200B' and 200C in the respective phases A, B and C of the three-phase power system 6I0 having a frequency fr continuouslyshow the relative phase difference of the corresponding phases of the two power systems 5|0 vand 6|0. With three vcarrier currents of different frequencies, it is possible to determine the phase relationship of a plurality of phases simul- `rier current for automatically compensating a three-phase-pilot line for phase change variav this arrangement the three-phase compenv sated pilot line is employed between stations A and B and each phase ofthis pilot line is com-- pensated simultaneously and continuously.J Separate automatic phase compensating units 99A, 99B and 99C are used with each phase of the pilot line |05 at station A which may each be similar to that shown in Fig. 13 for compensating a two-wire pilot line. At station B also sep'- arate automatic phase compensating equipment units 299A, 299B and 299C are used with each phase of the pilot line |05. The terminal apparatus at each station may also be similar to that shown in Fig.A 13 and is here schematically shown in Some detail for phase A at station A and `for phase C at station B. At station A oscillators 90A, 90B and 90C supply carrier frequencies for each phase of the compensating equipment. The' carrier frequencies for each phase -may be the same but it is preferable to use three different v frequencies'fz, .f3 and f4 for the three different phases respectively. The three phases of a power line 5|0 are properly connected to the terminal apparatus at station A and the three phases of a power line are properly connected to the terminal apparatus at station B. This frequency of the current on the power system is f1. At station B phase angle meters 200A, 200B and 200C continuallyshow thef phase relationship of the currents or voltages in the two power systems |0v and 6|0 for the respective phases of the two systems. In the drawing at station A three different oscillators have been shown each providing a different frequency carrier current. However, one oscillator providing a carrier curent of the same frequency for all three phases of the compensating equipment might be used as stated above but in this case each circuit of the pilot line should be better balanced than in the case where three different frequencies are used.

Fig. 18 showsan arrangement employing carrier current for automatically compensating one phase of a three-phase power line for phase change variations between two points'in which the compensation is achieved bytransmitting a modulated carrier current in opposite directions over the compensated one phase of the power line wires and arrangements at one point for making comparisons of phase relationship of currents or voltages at the two different points*` The terminal equipment in this arrangement is quite similar to that of Fig. 13 except that one phase of the power line is employed for the pilot transmitting medium instead of a separate pilot line. In either arrangement the pilot line circuit is used for other purposes, namely, in Fig. 13 it is used for telephony or other signaling, and in Fig. 18 it is also the circuit of one phase of the power system. Two of the wires or one phase of the three-phase power line |05 are used as the pilot impedance to the power vfrequency and a low ,impedance tothe carrier frequency, as heretofore stated. In addition to prevent reflection and to isolate the apparatus from the effects of switching on the power circuit, it is desirable to include at each end of the pilot line chokes |06 and |01, and 206 and 201, respectively, in series with the power line wires. Each choke may be tuned with a capacitance to make it anti-resonant to the pilot frequency. The automatic phase compensating apparatus at stations A a'nd B may be similar to that shown in Fig. 13 and is here so shown. Alternating current of frequency f1 from source |0 modulates in modulator 9| a carrier current of frequency ,f2 from oscillator 90 at station A and the sum (or difference) of these currents istransmitted over the pilot line and returned to cause the automatic compensation of the pilot line as explained in connection with Fig. 13. Alternating current of frequency f1 from source 2|0 at station B is impressed upon one side of the phase angle meter 200 and a part of the current transmitted from station A to station B is transmitted to demodulator 29| and in turn that having a frequency of f1 to the other side of the phase angle meter 200. 'Ihus the phase relationship of the currents or voltages generated at station A by source i0 and at station B by -source 2|0 are compared by the phase angle meter 200.

Fig. 19 shows an arrangement employing carrier current for automatically compensating one returning a harmonic frequency thereof over the compensated one phase of the power line wires and arrangements at one point for making comparisons of phase relationship of currents or voltages at the two diierentpoints. In this arrangement the automatic phase compensating equipment at station A and at station B is similar to that shown within thedot-dash boundary lines of Fig. 14 but arrangedto employ two wires of one phase of a polyphase power line for the pilot line between the two stations. The automatic compensating equipment at station A and at station B is connected to one phase of the power line |05 at each station by means of suitable coupling condensers. These condensers must withstand the power line voltages and at the same time provide a high impedance to the power frequencies and a low impedance to the carrier frequencies. 4At each station choke coils |06, |01 and |08, and 20G, 201 and 208, respectively, are inserted in series in the power line wires to prevent reilection and to isolate the terminal ap- .paratus from the effects of 'switching on the plained. At station B alternating current. of frecircuit. Coupling condensers connect the two must be capable of withstanding the power line voltages but at the same time ,provide a high '(5 quency f1 from-source 2|0 is impressed upon the phase angle meter 200, while a part of the current transmitted from station A after being demodulated in'demodulator 29| is impressed with a frequency of f1 upon the other side of the phase angle meter 200. The phase angle relationship of thealternating currents generated by the two sources is shown -by the phase angle meter.

' Fig. 20 shows an arrangement employing carrier current for automatically compensating one phase of a three-phase power line for phase change variations between two points in which compensation is achieved by transmitting a modulated carrier current in opposite directions over one phase of the power line between the two points and in which synchronous switching mechanism is used to connect in one'phase after another of the power circuits for making phase comparisons of the power currents or voltages of the. three-phase system at the two .different points. The automatic phase compensating equipment at each station A and B may be of any of the several types heretofore shown and as illustrated in this drawing figure it is similar to that shown in Fig. 13. This terminal equipment is connected at each station through suitable coupling condensers to two wires or one phase of the power line |05 and operates to automatically compensate this on'e circuit of the power line. Synchronous switching mechanism 3|0 and 410v at stations A and B, respectively, is arranged to connect in succession corresponding phases of the power system at the two stations. At station lA current from the phase of the power system which is connected through the synchronous switch. 310 modulates in modulator 9| a carrier current of frequency f2 generated by oscillator 90 andA carrier current components are produced for transmission over the two wires of the power system used as a pilot line to station .B and a partreturned for actuating theautomatic compensating apparatus las described more in detail in the description of Fig. 13. The different phasesof the power system or systems are connected as heretofore explained through suitable coupling condensers at each station to the respective synchronous switching mechanism. This synchronous switching mechanism may be of any well-known type. By this arrangement using a compensated one phase of the power line for a pilot line, the several phases of a power line or tw' different ,power systems transmitting currents having a frequency f1 may have the phase relationships of Atheir currents or voltages compared in the phase angle meter 200 at 'stationB by successivelyy connecting one phase after another ofthe power lines by-means of the synchronous switches.l y

Fig. 21 shows an arrangement employing carrier current for automatically compensating for all three phases of a `three-phase power line for phase change variations between two points in `whichcompensation is achieved by transmitting three different suitably modulated carrier currents in opposite directions over the power line wires of each phase between the two points and arrangements' at one point for simultaneously making comparisons of phase relationship of the currents or voltages in the different phases of the phases, A, B and C of the power line |05, respectively. Suitable choke coils are inserted in the power line wires to prevent reflection and to isolate the apparatus from the effect of switching on the power circuit as illustrated. The power line as here indicated is energized at each end with alternating current havinga frequency of f1 by suitable generators or input transformers, the details of which are not shown. The phase angle relationships of the two sources of power are continually shown in this arrangement by the phase angle meters 200A, 200B and 200C at station B. The three phases of the power line may be automatically compensated by any of the arrangements heretofore shown though the one illustrated is similar to that shown in Fig. 17. For each phase the power line current modulates a carrier frequency furnished by oscillators 90A, 90B and 90C of frequencies f2, fs and f4 for phases A, B and C, respectively.v

change variations at three frequencies between two points in which compensation is achieved by transmitting v three different modulated carrier current frequencies in opposite directions over thecompensated one phase of the' power linewires between the two points and arrangements power circuits at the two different points. This arrangement is 'somewhat similar to that shown in Fig. 17 in which the pilot line is a three-phase signal circuit while inl this case, the pilot line uses the circuits of a three-phase power system.

=- tective coupling condensers to the different at one point for simultaneously making comparisons of the phase relationship of the currents or voltages of the different phases of the three.

phase power circuits at the two different points. In Fig. 21 an arrangement was shown for lmaking phase relationship comparisons'simultaneously of all three phases of a power system between two different points by compensating all three phases of the power line'circuits and also using them as pilotline circuits. In Fig. 19 an arrangement is shown for usingv two wires or one phaseof a three-phasepower line as the compensated line for making phase relationship comparisons of currents or. voltages at two different points on all three phases of the power line. Fig. 22 compensates two wires or one circuit of the power line between two points at three different frequenciesl and provides for simultaneously making the phase relationship comparisons on all three phases of a power system or systems for two different points. The phase compensating equipment at each station A and B may be the same as shown for example in Fig. 19 where the lter type is used or the same asis used in Fig. 21

where the hybrid coil type is used. The auto? matic phase compensating apparatus 99A, 99B

and 09C at station A and 299A, 299Band 299C at station B are connected through suitable protective coupling condensers to one pair of wires or one phase of the power line |05.

erators or through input transformers, the de` sutiablechoke coils as previously described are inserted 

